One of the primary classes of artificial heart valves or prostheses is a “tissue-type” or “bioprosthetic” valve having flexible leaflets that function much like those of a natural human heart valve and imitate their natural action to coapt against each other and ensure one-way blood flow. In tissue valves, a whole xenograft valve (e.g., porcine) or a plurality of xenograft leaflets (e.g., bovine pericardium) typically provide fluid occluding surfaces. Synthetic leaflets have been proposed, and thus the term “flexible leaflet valve” refers to both natural and artificial “tissue-type” valves. Two or more flexible leaflets are mounted within a peripheral stent structure that usually includes posts or commissures extending in the outflow direction to mimic natural fibrous commissures in the native aortic annulus (the same construction may be used for the mitral annulus, though it does not correspond as closely with the mitral anatomy).
In most flexible leaflet valves, metallic or polymeric structure provides base support for the flexible leaflets, which extend therefrom. One such support is an elastic “support frame,” sometimes called a “wireform,” which has a plurality (typically three) of large radius cusps supporting the inflow cusp region of the leaflets of the bioprosthetic tissue (i.e., either a whole valve or three separate leaflets). The free ends of each two adjacent cusps converge somewhat asymptotically to form upstanding commissures that terminate in tips, each being curved in the opposite direction as the cusps, and having a relatively smaller radius. The wireform typically describes a conical tube with the commissure tips at the small diameter end. This provides an undulating reference shape to which a fixed edge of each leaflet attaches (via components such as fabric and sutures) much like the natural fibrous skeleton in the aortic annulus. Some valves include polymeric wireforms rather than metallic, for ease of manufacture or other reasons. For example, U.S. Pat. No. 5,895,420 discloses a plastic wireform that degrades in the body over time. In a hybrid construction, the CARPENTIER-EDWARDS Porcine Heart Valve and PERIMOUNT Pericardial Heart Valve available from Edwards Lifesciences of Irvine, Calif. both have ELGILOY wireforms surrounded by polymer bands.
U.S. Pat. Nos. 4,035,849 to Angell, et al., 4,388,735 to Ionescu, et al., and 4,626,255 to Reichart, et al. disclose various flexible leaflet prosthetic heart valves with fabric-covered stents. Another example of the construction of a flexible leaflet valve is seen in U.S. Pat. No. 5,928,281 to Huynh, et al., in which the exploded view of FIG. 1 illustrates a support stent comprising a fabric-covered wireform and a fabric-covered stent on either side of a leaflet subassembly. FIGS. 1A and 1B are elevational and top plan views, respectively, of an exemplary support stent 18 for a prosthetic heart valve similar to that disclosed in the patent to Huynh, et al. The support stent includes multiple cusps curved toward an axial inflow end alternating with multiple commissures projecting toward an axial outflow end, the support stent 18 defining an undulating outflow edge. The illustrated support stent 18 comprises a wireform 20 having three upstanding commissures 22 alternating with three cusps 24 of larger radii which generally circumscribe a tube. A circular supporting band 26 closely surrounds the wireform 20 and defines an inflow edge 28 and an outflow edge 30. The inflow edge of the band 26 conforms to the cusps 24 of the wireform 20, and may be curved in the outflow direction in between in the region of the wireform commissures 22. The outflow edge 30 extends approximately halfway up the wireform commissures 22, and dips down therebetween in the inflow direction. This type of support stent 18 forms the structural “spine” of a one-way prosthetic heart valve which may be implanted in any of the four orifices in the heart, though most commonly in either the mitral or aortic positions.
Components of the valve are usually assembled with one or more biocompatible fabric (e.g., Dacron, polyethylene terepthalate) coverings, and a fabric-covered sewing ring is provided on the inflow end of the stent. The fabric coverings provide anchoring surfaces for sutures to hold the flexible leaflets and sewing ring to the peripheral stent structure. In one of the assembly procedures, a tubular fabric is tightly wrapped around the undulating stent and sewn closed with one or more peripheral seams. Because of the undulating shape of the support stent, the process involves manually holding a tube of fabric around the stent and the sewing is typically accomplished in two stages; first, intermittent stitches are placed to secure the fabric in its gross position around the stent, and then a closely-spaced line of stitches is applied to complete the seam. The odd shape of the stent and the presence of two or more components being enclosed by the fabric cover necessitates that the holding and stitching operation is done manually, which makes it quite labor-intensive and time-consuming. Quality control in the manufacture of heart valves further increases the difficulty of the task because the fabric must be tightly fitted around the stent. This manual sewing procedure represents a substantial portion of the cost of the entire valve fabrication process. Furthermore, repetitive stress injuries can occur which is painful to the worker and indirectly increases the cost of making the valve. Indeed, most of the steps in assembling prosthetic heart valves are specialized, manual tasks performed in a clean room. Typically the components are held by the worker and sewn together at the same time, which is a laborious process considering the exacting nature of the quality control applied to the subsequent finished product.
There is thus a need for an improved method for assembling flexible heart valves that reduces the assembly time and reduces the instances of injury to the assembly-line workers.